In the last decade, numerous schemes have been proposed for the imperceptible embedding and detection of marks in signals. For a mark to be imperceptible in a signal, such as an image or even a video, the mark is typically embedded at a very low level when compared with the signal. For such a low-level signal to be detectable, a highly efficient detection process is needed. Such a process exists in the form of correlation or “matched filtering” in the case where the process is an optimal linear detection.
Certain marks have near perfect correlation properties, allowing virtually all the energy of the mark to be concentrated into a single detection point. Unfortunately most correlation processes are effectively destroyed, or at least severely impeded, by distortions of the marked signal. In two dimensions, common distortions include rotation, scaling, anamorphic scaling and shear.